This invention relates to a silver halide photographic material containing at least one silver halide emulsion that has enhanced light absorption and high photographic sensitivity.
J-aggregating cyanine dyes are used in many photographic systems. It is believed that these dyes adsorb to a silver halide emulsion and pack together on their xe2x80x9cedgexe2x80x9d which allows the maximum number of dye molecules to be placed on the surface. However, a monolayer of dye, even one with as high an extinction coefficient as a J-aggregated cyanine dye, absorbs only a small fraction of the light impinging on it per unit area. The advent of tabular emulsions allowed more dye to be put on the grains due to increased surface area. However, in most photographic systems, it is still the case that not all the available light is being collected.
The yellow, magenta, and cyan layers in many color photographic materials could benefit from increased light absorption which could allow the use of smaller emulsions with less radiation sensitivity and improved color and image structure characteristics. For certain applications, it may be useful to enhance infrared light absorption in infrared sensitized photographic elements to achieve greater sensitivity and image structure characteristics.
One way to achieve greater light absorption is to increase the amount of spectral sensitizing dye associated with the individual grains beyond monolayer coverage of dye (some proposed approaches are described in the literature, G. R. Bird, Photogr. Sci. Eng., 18, 562 (1974)). One method is to synthesize molecules in which two dye chromophores are covalently connected by a linking group (see U.S. Pat. No. 2,518,731, U.S. Pat. No. 3,976,493, U.S. Pat. No. 3,976,640, U.S. Pat. No. 3,622,316, Kokai Sho 64(1989)91134, and EP 565,074). This approach suffers from the fact that when the two dyes are connected they can interfere with each other""s performance, e.g., not aggregating on or adsorbing to the silver halide grain properly.
In a similar approach, several dye polymers were synthesized in which cyanine dyes were tethered to poly-L-lysine (U.S. Pat. No. 4,950,587). These polymers could be combined with a silver halide emulsion, however, they tended to sensitize poorly and dye stain (an unwanted increase in D-min due to retained sensitizing dye after processing) was severe in this system and unacceptable.
A different strategy involves the use of two dyes that are not connected to one another. In this approach the dyes can be added sequentially and are less likely to interfere with one another. Miysaka et al. in EP 270 079 and EP 270 082 describe silver halide photographic material having an emulsion spectrally sensitized with an adsorbable sensitizing dye used in combination with a non-adsorbable luminescent dye which is located in the gelatin phase of the element. Steiger et al. in U.S. Pat. No. 4,040,825 and U.S. Pat. No. 4,138,551 describe silver halide photographic material having an emulsion spectrally sensitized with an adsorbable sensitizing dye used in combination with a second dye that is bonded to gelatin. The problem with these approaches is that unless the dye not adsorbed to the grain is in close proximity to the dye adsorbed on the grain (less than 50 angstroms separation) efficient energy transfer will not occur (see T. Fxc3x6rster, Disc. Faraday Soc., 27, 7 (1959)). Most dye off-the-grain in these systems will not be close enough to the silver halide grain for energy transfer, but will instead absorb light and act as a filter dye leading to a speed loss. A good analysis of the problem with this approach is given by Steiger et al. (Photogr. Sci. Eng., 27, 59 (1983)).
A more useful method is to have two or more dyes form layers on the silver halide grain. Penner and Gilman described the occurrence of greater than monolayer levels of cyanine dye on emulsion grains, Photogr. Sci. Eng., 20, 97 (1976); see also Penner, Photogr. Sci. Eng., 21, 32 (1977). In these cases, the outer dye layer absorbed light at a longer wavelength than the inner dye layer (the layer adsorbed to the silver halide grain). Bird et al. in U.S. Pat. No. 3,622,316 describe a similar system. A requirement was that the outer dye layer absorb light at a shorter wavelength than the inner layer. The problem with previous dye layering approaches was that the dye layers described produced a very broad sensitization envelope. This would lead to poor color reproduction since, for example, the silver halide grains in the same color record would be sensitive to both green and red light.
Yamashita et. al. (EP 838 719 A2) describes the use of two or more cyanine dyes to form more than one dye layer on silver halide emulsions. The dyes are required to have at least one aromatic or heteroaromatic substituent attached to the chromophore via the nitrogen atoms of the dye. Yamashita et. al. teaches that dye layering will not occur if this requirement is not met. This is undesirable because such substitutents can lead to large amounts of retained dye after processing (dye stain) which affords increased D-min. We have found that this is not necessary and that neither dye is required to have at least one aromatic or heteroaromatic substitute attached to the chromophore via the nitrogen atoms of the dye.
Not all the available light is being collected in many photographic systems. The yellow, magenta, and cyan layers in many photographic products could benefit from increased light absorption which could allow the use of smaller emulsions with less radiation sensitivity and improved color and image quality characteristics. For certain applications, it may be useful to enhance infrared light absorption in infrared sensitized photographic elements to achieve greater sensitivity and image quality characteristics.
In some cases the excited state lifetimes of dyes are too short to allow efficient energy transfer between dye layers. For example, dyes that absorb red light and that could be used in an additional dye layer often have excited state lifetimes that are very short. This is indicated by a weak fluorescence when the dye is aggregated in aqueous gelatin. We have found that red-light absorbing dyes that aggregate in aqueous gelatin and have a fluorescent signal above a certain threshold can give enhance photographic sensitivity when used to form a second layer on a red spectrally sensitized silver halide emulsion.
We have found that it is possible to form more than one dye layer on silver halide emulsion grains and that this can afford increased light absorption. The dye layers are held together by a non-covalent attractive force such as electrostatic bonding, van der Waals interactions, hydrogen bonding, hydrophobic interactions, dipole-dipole interactions, dipole-induced dipole interactions, London dispersion forces, cation-xcfx80 interactions, etc. or by in situ bond formation. The inner dye layer(s) is adsorbed to the silver halide grains and contains at least one spectral sensitizer. The outer dye layer(s) (also referred to herein as an antenna dye layer(s)) absorbs light at an equal or higher energy (equal or shorter wavelength) than the adjacent inner dye layer(s). The light energy emission wavelength of the outer dye layer overlaps with the light energy absorption wavelength of the adjacent inner dye layer.
We have also found that silver halide grains sensitized with at least one dye containing at least one anionic substituent and at least one highly fluorescent dye containing at least one cationic substituent provides increased light absorption.
One aspect of this invention comprises a silver halide photographic material comprising at least one silver halide emulsion comprising silver halide grains having associated therewith at least two dyes:
(a) a first dye that has at least one anionic substituent and that is capable of spectrally sensitizing a silver halide emulsion; and
(b) a second dye that absorbs light at an equal or shorter wavelength than the first dye and that has at least one cationic substituent and that is highly fluorescent.
The dyes of this invention preferably form a J-aggregate in aqueous gelatin. The J-aggregate formed preferably is highly fluorescent. It preferably has a fluorescence intensity above a certain threshold, as discussed in more detail below. The dyes also preferably have liquid-crystalline properties in aqueous gelatin. The dyes used in this invention afford improved spectral sensitivity.
Generally the first dye forms an inner dye layer around the silver halide grain and the second dye forms an outer dye layer. Either or both of the layers may contain one or more additional dyes.
The second dye preferably absorbs light at an equal or shorter wavelength than the first dye. The wavelength of maximum light absorption of the second dye can be equal to or slightly deeper than the wavelength of maximum light absorption of the first dye provided there is still substantial overlap between the light absorption envelope of the second dye and the light absorption envelope of the first dye. More preferably, the wavelength of maximum light absorption of the second dye is shorter than the wavelength of maximum light absorption of the first dye. In preferred embodiments of he invention the wavelength of maximum light absorption of the second dye is between 5 and 100 nm, more preferably between 5 and 50 nm, and most preferably 5 to 30 nm, shorter than the wavelength of maximum light absorption of the first dye.
The invention provides increased light absorption and photographic sensitivity by forming more than one layer of sensitizing dye on silver halide grains. The increased sensitivity could be used to improve granularity by using smaller emulsions and compensating the loss in speed due to the smaller emulsions by the increased light absorption of the dye layers of the invention. In addition to improved granularity, the smaller emulsions would have lower ionizing radiation sensitivity. Radiation sensitivity is determined by the mass of silver halide per grain. The invention also provides good color reproduction, i.e., no excessive unwanted absorptions in a different color record. This invention achieves these features whereas methods described in the prior art can not.